1. Field of the Invention
This invention relates to conductive paint formulations which are useful for coating housings for electronic equipment to provide electromagnetic interference (EMI) and radio frequency interference (RFI) shielding for the housings and which are also useful in making printing plates and other printing devices to provide a durable, ink transferable surface on the printing plate and, in particular, to paint formulations which may be spray coated onto the housing or printing plate to provide a thin durable finish which is both adhesive to the housing substrate and cohesive with the conductive pigment and which has low electrical resistivity.
2. Description of Related Art
It is often necessary to provide EMI/RFI shielding for polymeric and other electrically non-conductive enclosures used in electronic and radio equipment to prevent radiation such as EMI/RFI from being emitted from the equipment contained therein. Regulations of the Federal Communication Commission require that certain categories of electronic and computer equipment be enclosed by shields capable of substantially eliminating such EMI/RFI above about 10 kiloHertz (kHz) in frequency.
It is known in general that electrically conducting materials will provide EMI/RFI shielding and, accordingly, it is common practice in the industry to provide a metal coating on enclosures for electronic equipment. Among the techniques known to apply such a coating are the use of electrically conductive paints, flame or arc metal spraying, the application of metal foils and electroless or electrolytic deposition of one or more metal layers.
The many types of electronic equipment which must be shielded from EMI/RFI radiation include commercial, industrial, business and home products such as computers, calculators, video equipment and electronic games. The use of conductive finishes such as electrically conductive paints on the electronic housing however are subject to failure by abrasion, flaking, cracking or corroding allowing leaking of radiation. For this and other reasons, multiple coatings are typically applied for many applications to provide acceptable shielding properties.
To evaluate the electrical effectiveness of a conductive coating on the electronic housing, a test of surface resistivity-measured in ohm/squarexe2x80x94termed xe2x80x9cohms per squarexe2x80x9d is typically used. Both point to point and surface resistivities are usually measured using standard electronic measuring instruments. The durability of the finish and its adhesion to the electronic component substrate is also an important factor in determining the effectiveness of the coating. The conductive paints used to form conductive coatings employ a metallic additive (pigment) in the paint to provide the conductivity of the coating and the ability of the coating to retain the metallic additive (typically termed xe2x80x9cmetallic pigmentxe2x80x9d or xe2x80x9cpigmentxe2x80x9d) is also a measure of the effectiveness of the coating and is usually termed xe2x80x9ccohesionxe2x80x9d.
Metal containing conductive paints are very popular from an industrial viewpoint to provide a conductive coating on electronic component housings for shielding purposes because of their cost effectiveness. In general, a conductive paint formulation contains a metal such as nickel, copper, and preferably silver and combinations with silver to provide electrical conductivity, a solvent and a binder of generally a polymeric material which are thermoplastic and which on drying form a hard conductive film. The conductive paints are typically applied by spraying with conventional painting equipment and the coated housings are then usually air-dried. Silver has the highest conductivity and is considered the best type of pigment to use for the most critical applications. The high cost of silver however makes it unattractive for general purpose use. Copper is another metal used in paint but oxidation during use results in loss of conductivity and limits its application. A silver coated copper particle, preferably in the form of flake, has been proposed for use in conductive paints alone as the metal pigment and in admixture with pure silver flake pigment.
At present, conductive paints because of their ease of use and effectiveness are very desirable to use in shielding housings for electronic equipment. There are a number of advantages to the use of conductive paints such as minimal equipment cost, ease of application, and low material cost relative to other conductive finishes. However, coating durability is a significant concern since cracks or chips in the coating or abrasion of the coating will effect the shielding of the coating. The quality of the coating therefore is very important to the shielding effectiveness of the coating. Also, in applying the paint it is usually necessary to employ an agitator or mechanical shaker to keep the relatively heavy metallic particles from settling so a homogeneous mixture is maintained during spray application. In general, for a conductive metal paint, and particularly a conductive paint which is sprayed, it is important that the spray coating be durable, have a high conductivity (low resistivity) and provide a smooth (even) coating. It is also important from a cost and application standpoint that a thin coating be used. Paints even meeting these criteria must also have adhesive and cohesive strength so that the coating adheres to the substrate and the conductive pigment is retained in the paint film. All these properties are now typically balanced because a change in the formulation to achieve a particular result typically has a negative effect on another property. For example, increasing the metallic loading decreases the resistivity but usually causes the paint to have poor cohesion.
In U.S. Pat. No. 5,158,708 an electrically conductive paste is described for screen printing of printed circuit boards to provide an electromagnetic shielding layer. The paste is not used for spray painting of housings for shielding. In U.S. Pat. No. 5,282,887 conductive paints applied by spraying are disclosed containing a pigment grade carbon, a resin, a cross-linking agent and a solvent. U.S. Pat. No. 4,305,847 discloses copper containing compositions for shielding employing organic titanates to stabilize the paint over a long period of time even at elevated temperatures.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an electrically conductive metallic containing paint for EMI/RFI shielding for housings of electronic equipment which paint provides a durable coating with a low resistivity at thin coatings as low as 0.2 mils and has both excellent adhesion to the substrate and cohesive properties of the coating.
It is another object of the present invention to provide an electrically conductive silver containing paint for EMI/RFI shielding for housings of electronic equipment which paint provides a durable coating with a low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is a further object of the present invention to provide an electrically conductive silver coated/copper particle containing paint, preferably in admixture with silver particles, for EMI/RFI shielding for housings of electronic equipment which paint provides a durable coating with low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is a further object of the present invention to provide a method for forming an EMI/RFI shielding on substrates such as housings for electronic equipment which method provides a metal coating, preferably a silver paint coating which is durable, has low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is an additional object of the present invention to provide a method for forming an EMI/RFI shielding on substrates such as housings for electronic equipment which method provides a silver coated copper particle containing conductive paint, preferably in admixture with silver particles, which conductive paint provides a coating which is durable, has low resistivity at thin coatings and has both excellent adhesive and cohesive properties.
It is another object of the present invention to provide a method for forming an EMI/RFI shielding on substrates such as housings for electronic equipment which method provides a coating which is durable, has low resistivity and provides a smooth finish.
It is a further object of the present invention to provide EMI/RFI shielded housings for electronic equipment which are made using the conductive paints and methods of the present invention.
It is an additional object of the present invention to provide a method for making a printing plate or other printing device useful in the printing or graphic design arts for printing ink designs on substrates and articles such as beverage cans, etc., which printing plates or printing devices are made using the paint and method of the invention.
In another object of the invention, a printing plate is provided which has an enhanced operating life and ink transfer surface.
Another object of the invention is to provide a method for printing patterns on substrates and articles such as beverage cans using the printing plate of the invention.
It is a further object of the invention to provide printed articles, substrates and the like such as beverage cans, etc. which are made using printing plates or other printing devices made using the paint and method of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, an electrically conductive metal containing paint for EMI/RFI shielding for housings of electronic equipment comprising an organic binder resin having crosslinkable functional groups selected for example from the group consisting of xe2x80x94OH, xe2x80x94COOH, xe2x80x94CONH2, xe2x80x94SH, phenolics, xe2x80x94NH2 and xe2x80x94NHCH2OH; electrically conductive metallic particles preferably silver and silver-coated copper flakes and mixtures thereof; a solvent; an effective amount of a cross-linking agent which cross-links with itself and with the functional groups of the organic binder resin; and highly preferably an acid or alkaline catalyst to accelerate cross-linking of the organic binder resin with the cross-linking agent. The use of a binder resin having cross-linkable functional groups in combination with a cross-linking agent and preferably a catalyst to catalyze the cross-linking reaction, has been found to provide a metal containing conductive paint for EMI/RFI shielding which may be applied in thin coatings as low as 0.2 mils yet still have the abrasion resistance (durability), low resistivity and adhesive and cohesive properties needed for EMI/RFI shielding applications and other applications such as the making of printing plates used to ink print articles such as beverage cans whereby the printing plates are made by spraying or applying the conductive paint of the invention to the printing plate.
In another aspect of the invention, an electrically conductive silver containing paint for EMI/RFI shielding for housings of electronic equipment comprises an organic binder resin having crosslinkable functional groups selected for example from the group consisting of xe2x80x94OH, xe2x80x94COOH, xe2x80x94CONH2, xe2x80x94SH, phenolics, xe2x80x94NH2 and xe2x80x94NHCH2OH; electrically conductive silver particles, preferably flakes; a solvent; an effective amount of a cross-linking agent which cross-links with itself and with the functional groups of the organic binder resin; and highly preferably an effective amount of an acid or alkaline catalyst to accelerate cross-linking of the organic binder resin with the cross-linking agent.
In another aspect of the invention, an electrically conductive silver coated copper particle (preferably flake) containing paint for EMI/RFI shielding for housings of electronic equipment comprises an organic binder resin having crosslinkable functional groups selected for example from the group consisting of xe2x80x94OH, xe2x80x94COOH, xe2x80x94CONH2, xe2x80x94SH, phenolics, xe2x80x94NH2 and xe2x80x94NHCH2OH; electrically conductive silver coated copper particles, preferably flakes; a solvent; an effective amount of a cross-linking agent which cross-links with itself and with the functional groups of the organic binder resin; and highly preferably an effective amount of an acid or alkaline catalyst to accelerate cross-linking of the organic binder resin with the cross-linking agent. This paint preferably also contains silver flake in the paint formulation.
In another aspect of the invention, the above paint formulations preferably contain a rheological additive in the paint which rheological additive is highly preferably an organic derivative of castor oil particularly an organic castor oil derivative termed castor wax.
In an additional aspect of the invention, the organic binder resin is a poly (vinyl alcohol-co-vinyl acetate-co-vinyl chloride) terpolymer having a molecular weight of about 15,000 to 25,000 and contains about, by weight %, 5-6 vinyl alcohol content (secondary alcohol), 90 vinyl chloride content and 4 vinyl acetate ester content. The cross-linking agent is preferably a melamine resin which cross-links with itself and also with the hydroxyl groups of the organic binder resin. It is preferred to use an acid catalyst with the cross-linking agent to accelerate the cross-linking of the formulation and it is highly preferred to use an organic acid such as paratoluenesulfonic acid or methanesulfonic acid as the catalyst . It is also preferred to use an anti-settling agent in the above paints to control settling of the metallic particles and the forming of a paint condition known as hard pack.
In another aspect of the invention, is has been found that the catalysis of the cross-linking process be specially controlled depending on the type metallic particles used in the paint formulation. For paints containing pure silver particles, the amount of catalyst is specially controlled to achieve the desired combination of EMI/RFI shielding vis-a-vis the coating thickness. For paint formulations containing silver coated copper particles control of the amount of catalyst used is not as restricted but is still specially controlled and a wider catalyst compositional operating range may be used for most shielding applications compared to pure silver paint formulations.
In another aspect of the invention, the conductive metal paint is preferably prepared as a two component system. In the first component the resin is dissolved in a solvent and then additives, if any, such as a Theological additive and an anti-settling agent, are added using a high speed mixer disperser to homogenize the ingredients. After homogenization, the other additives are added such as the metallic pigments and more solvent if necessary. The metal pigment addition is preferably performed under low speed mixing to prevent distorting of the metallic particles. The cross-linking component is then preferably added as the last component. This composition may be termed xe2x80x9cpaint component 1xe2x80x9d. The second paint component comprises an acid or alkaline catalyst for accelerating the cross-linking reaction and a solvent. When the paint is desired to be used, paint component 1 is mixed with paint component 2 and a paint is provided which typically has a pot life of about one day which varies depending on the temperature conditions of use. Additional solvent is usually added to the mixture of paint component 1 and paint component 2 to provide the desired paint viscosity for spraying or other coating applications. The paint is typically sprayed at a temperature of about 20-30xc2x0 C. Further, retarder solvents may be added to the paint formulation to minimize dry spray which results in loose non-adherent particles on the sprayed surface. Solvents such as butyl cellosolve and diacetone alcohol may be used to slow the evaporation rate of the paint during spraying thus minimizing dry spray.
In another aspect of the invention, a method is provided for applying the paint of the invention to a substrate to make articles of manufacture including EMI/RFI shielded housings for electronic components and printing plates.
In a further aspect of the invention, a method is provided for forming an EMI/RFI shielding for housing for electronic equipment comprising applying an electrically conductive metal paint thereto as defined hereinabove in a shielding effective thickness. It is preferred that the paint be applied by spraying although the paint may be applied by immersion, brushing, etc.
In a further aspect of the invention an EMI/RFI shielded substrate, and in particular, a shielded housing for electronic equipment, is provided which is made by the method of the invention using the various paint formulations of the invention.
To provide a paint formulation needed to coat housing for electronic equipment to provide EMI/RFI shielding, it is conventional to control the pigment to binder ratio to provide the desired properties such as durability, adhesion, cohesion, and low resistivity. It has now been found that the amount of resin used in the paint formulation may be reduced or minimized in a conductive metal paint formulation thereby increasing the pigment/binder ratio while increasing or exceeding the properties of prior art conductive metal paint formulations. The reduction of the amount of the binder enables the use of thinner coatings yet the paint formulation of the invention still exceeds the desired low resistivity values and provides a significantly more durable finish than conventional paints and which paint finish has excellent adhesive and cohesive properties. The use in a metal containing paint formulation of a cross-linking agent in combination with a binder resin having functional groups cross-linkable with the cross-linking agent and catalysis of the binder cross-linking agent reaction has been found to provide a paint formulation in which the amount of resin in the paint formulation can be minimized while increasing or exceeding at least some of the shielding properties of prior art conductive metal paints.
In a further aspect of the invention a method is provided for making a printing plate or other printing device which printing plate has a patternable layer on the plate, typically an imageable polymer, with the patternable layer patterned to form a patterned layer, which patterned layer has a transfer ink applied thereto and the ink is transferred from the patterned layer in the patterned design to an article or substrate to be printed such as a beverage can, the method comprising the steps of:
coating a patterned substrate with a paint of the invention, the substrate preferably having a coating of an imageable polymer thereon which polymer has been imaged with the pattern;
curing the paint to form a printing plate or device having a durable, ink transferable patterned surface.
In a further aspect of the invention a method is provided for printing with a printing plate or other printing device which printing plate has a patternable layer on the plate, typically an imageable polymer, with the patternable layer patterned to form a patterned layer, which patterned layer has a transfer ink applied thereto and the ink is transferred from the patterned layer in the patterned design to an article or substrate to be printed such as a beverage can, the method comprising the steps of:
applying printing ink to a printing plate of the invention covering a patterned layer on the printing plate;
transferring the ink from the patterned layer of the printing plate to a substrate or article to be printed with the pattern and printing the pattern on the substrate; and
repeating, if necessary, the steps for applying printing ink to one or more other printing plates and transferring the ink to the substrate to make desired patterns on the substrate which plates are used sequentially to provide multiple ink coatings on the substrate to make the final patterned product.
In a further aspect of the invention a method is provided for making a printing plate or other printing device which printing plate has a patternable layer on the plate with the patternable layer patterned to form a patterned layer, which patterned layer has a transfer ink applied thereto and the ink is transferred from the patterned layer in the patterned design to an article or substrate to be printed such as a beverage can, the method comprising the steps of:
supplying a substrate preferably metallic and preferably in the form of a sheet which is bendable;
preferably applying an adherent layer which adheres to the substrate, preferably polymeric, and which provides an adhesive surface;
coating the substrate or preferably a substrate containing an adherent layer with an imageable material such as a polymer forming an imageable (or patternable) layer of the material;
patterning the imageable layer by removing portions of the imageable layer and/or forming openings in the imageable layer with the patterned layer forming the desired pattern to be transferred to an article;
coating the patterned layer containing substrate with a paint of the invention; and
curing the paint and forming a printing plate or device having a durable ink transferable patterned surface.
In another aspect of the invention, a printing plate or device is provided which is made using the paint of the invention comprising a substrate having a patterned layer thereon, and a paint coating on the patterned layer which paint coating has been cured, the printing plate having enhanced operating life and ink transferable patterned surface.
In a further aspect of the invention graphic design ink printed articles such as beverage cans, toys, and printed matter and the like are made using the method and printing plate or device of the invention.